You can't use it as-is, because it uses defined-or ( $var //= 'new'; ), a perl-5.10 feature. Here's a patch to fix that

--- perl5db.pl    2007-12-18 11:38:11.000000000 -0800
+++ perl5dbFOR5.8.pl    2008-10-17 18:00:50.812500000 -0700
@@ -6025,7 +6025,7 @@
 } ## end sub setterm
 
 sub load_hist {
-    $histfile //= option_val("HistFile", undef);
+    $histsize = option_val("HistFile",undef) unless defined $hintsize
+;#patch-5.8 $histfile //= option_val("HistFile", undef);
     return unless defined $histfile;
     open my $fh, "<", $histfile or return;
     local $/ = "\n";
@@ -6043,7 +6043,7 @@
     eval { require File::Basename } or return;
     File::Path::mkpath(File::Basename::dirname($histfile));
     open my $fh, ">", $histfile or die "Could not open '$histfile': $
+!";
-    $histsize //= option_val("HistSize",100);
+    $histsize = option_val("HistSize",100) unless defined $hintsize;#
+patch-5.8 $histsize //= option_val("HistSize",100);
     my @copy = grep { $_ ne '?' } @hist;
     my $start = scalar(@copy) > $histsize ? scalar(@copy)-$histsize :
+ 0;
     for ($start .. $#copy) {
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